# Telling time with Jupiters moons?

I was watching the Nova show, "Lost at Sea; The Search for Longitude", which describes our quest to accuately determine our longitude as we travel across the sea. It said that watching the moons of Jupiter was an accurate way of keeping the time of your home port but wasn't practical to do on the rolling ocean. This is fascinating! Can anyone either explain to me how to use Jupiter's moons as a timepiece or direct me to another resource?

I'd think the fluctuations in each moon's orbit would create inconsistencies. It seems like jupiters relative position to Earth would create inconsistencies, too. Any insights?

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## Answers and Replies

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russ_watters
Mentor
Any periodic/circular motion can be used to tell time and our solar system provides us with a lot of examples: the rotation of the earth giving a repeating pattern of the Sun's position being a common one.

So if you know the starting position of a moon and it's period of revolution, you can count cycles and multiply to get elapsed time, just as we do with days and the sun. Now if you lose track, that can be a problem, but fortunately, Jupiter has 4 bright moons with different periods and by charting them all, you can find a large number of unique patterns that you can simply occasionally check to get the date.

Getting the time and not just the date is simply a matter of precision. Transits of moons across the disk can be predicted and viewed to within a few minutes. I suspect that the biggest difficulty would be in keeping the planet in your field of view at high magnification to view the event.

http://www.topogs.org/jupiter.htm

Thanks for the fun link! So you don't think my 16 power binocs are enough to recognize the transits? What about a 100 power spotting scope?

Here's part of what I don't understand: The way to figure out how far you are from the home port is to compare local time with home port time. The only way to know local time is by taking a sundial reading at noon. But you can't see Jupiter at noon. You have to wait several hours and by that time you don't know local time anymore.

At any rate, I'd like to begin tracking Jupiter's moons so I can better understand this. Does anyone know where I can find tables to get me started? Ideally, they would be in some sort of book that explains how to use them. I'm hoping the U.S. Naval Observatory's annual Astronomical Almanac isn't my only option.

russ_watters
Mentor
Thanks for the fun link! So you don't think my 16 power binocs are enough to recognize the transits? What about a 100 power spotting scope?
16 power is enough to see them, but the magnification is too low to pinpoint the transits/occultations to closer than 10 or 20 minutes, I'd think. 100 power is pretty good for Jupiter.
Here's part of what I don't understand: The way to figure out how far you are from the home port is to compare local time with home port time. The only way to know local time is by taking a sundial reading at noon.
Any celestial object crossing a known meridian at a known time will indicate the longitude. Philadelphia is almost exactly 5 hours behind GMT, so the sun will cross the meridian 5 hours later.....and Jupiter will also cross it 5 hours later for Philly than for Greenwich.

So you can use the transit timing synchronize a clock and the position in the sky at that time to fix the longitude....or just measure the time from then until the meridian crossing of either Jupiter or the sun to fix the longitude. It depends on what tools and tables are available to you.
At any rate, I'd like to begin tracking Jupiter's moons so I can better understand this. Does anyone know where I can find tables to get me started? Ideally, they would be in some sort of book that explains how to use them. I'm hoping the U.S. Naval Observatory's annual Astronomical Almanac isn't my only option.
Depends on how accurate you want to get -- I'm sure someone has tables, but this little, old applet will give you positions to within an hour or two: http://www.softlookup.com/display.asp?id=2072

DaveC426913
Gold Member
One of the first things I did with my scope was plot the positions of the Galilean moons over the course of a couple of weeks. Very cool.

Looks like a quadruple helix curling around a cylindrical Jupiter. Each helix has...

Oh wait. A picture is worth a thousand words.

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Any celestial object crossing a known meridian at a known time will indicate the longitude. [...] So you can use the transit timing synchronize a clock and the position in the sky at that time to fix the longitude
True. But what if neither the meridian nor the time are known? Before clocks were reliable enough to take across the ocean, neither of these factors could be known! As far as I know, without knowing the longitude and without a reliable timepiece, the only way to know the time was by a noon sundial reading.

Apparently, if you landed on some distant shore (as opposed to being on a rolling boat), you could use Jupiter's moon to determine home port time, and compare it with local time to find longitude. But I don't see how this is possible without an accurate timepiece, unless you can determine local time at night, when you could simultaneously see Jupiter. Apparently, however, it is possible. There's just something about the technique that I don't know.

DaveC, it's amazing how nice those simple pleasures can be. You don't NEED to plot the moons. You can just get a table! But there is pleasure to seeing it in action for yourself. Part of my problem right now (certainly the least of them) is that I don't know which moon is which, so I wouldn't know which moon I was tracking. Supposedly the moons are very accurate for keeping time, though, so one should somehow be able to plot the movements by minutes.

DaveC426913
Gold Member
True. But what if neither the meridian nor the time are known?
You miss the point.

Your records (from port) predict when the transit will occur. Thus, when you see the transit, you know what time it is back at port. Now you can calculate the meridian.

You miss the point.

Your records (from port) predict when the transit will occur. Thus, when you see the transit, you know what time it is back at port. Now you can calculate the meridian.
I'm really not missing the point. You can only make the aforementioned calculation if you know local time. It's the comparison between local time and port time that allows you to calculate the meridian. But when you're sailing in the middle of nowhere, with nothing around for a frame of reference, it can be hard to know if you've traveled 50 miles or 150 miles. Without knowing how far you've traveled, you can't know the local time. Back in the day, finding the meridian was a real problem.

Evidentally, however, it wouldn't have been a problem if the boat could be steady enough to view the moons of Jupiter. But this was an impossible task, so they had to find other methods. That method ended up being the creation of a highly accurate pocketwatch, by a self-taught, and otherwise uneducated clock maker from a small village named John Harrison.

I just thought it would be neat to learn how to use "pre-clock" knowlege to discover how far you've traveled. But the ellapsed time between a noon sun reading and an evening/night time Jupiter reading has me perplexed. I just hoped someone on the Forums might have insights. But maybe we're all perpelxed.

D H
Staff Emeritus
Without knowing how far you've traveled, you can't know the local time..
Calculating local time is trivial. Determine local noon, determine the time when some reference star rises or sets, etc. The problem was always having a reference time. A precise clock will do it, but so will observations of the moons of Jupiter.

DaveC426913
Gold Member
...it can be hard to know if you've traveled 50 miles or 150 miles. Without knowing how far you've traveled...
Let's keep our eye on the ball.

Knowing how far you've travelled (and any other aspect of one's geographical location) is the goal of the entire exercise - the ability to draw accurate maps.

We only care about the time because it allows us to figure out where we are.

Calculating local time is trivial. Determine local noon, determine the time when some reference star rises or sets, etc. The problem was always having a reference time. A precise clock will do it, but so will observations of the moons of Jupiter.
Thanks for your input D.H. Unfortunately, I'm ready to give up finding help with this problem. If you re-read the middle paragraph of post #3, as well as my subsequent posts, you'd see that my question is how one reconciles local time - by a noon sundial reading - with a reading of the moons of Jupiter, which needs to happen much later. The inability to keep track of time between the two readings is where the method breaks down for me.

A noon reading is certainly the most precise method for determining local time. Rising or setting stars may also work, but they change slightly each day as well as with even small latitude changes, which will always introduce a greater margin of error.

At any rate, thanks to everyone for their efforts and especially to Russ for his great links.

DaveC426913
Gold Member
The inability to keep track of time between the two readings is where the method breaks down for me.
A simple hourglass will do that.

The missing piece has always been accurate timekeeping over long periods.

A noon reading is certainly the most precise method for determining local time. Rising or setting stars may also work, but they change slightly each day as well as with even small latitude changes, which will always introduce a greater margin of error.
Easily calculated.

I think you undestestimate how important, and thus how much manpower and money was invested in navigation during this period of discovery. No expense would have been spared, no discerning mind ignored, no table of calculations unpublished.

If it took a sailor standing on deck for 12 hours, turning over and hourglass every hour, or a mathematician publishing tables that show exactly the variations in the sky over the seasons - those are trivial compared to a couple of loaded galleons lost at sea, or foundering on rocks because their maps are inaccurate.

I think if you kept a calendar of Jupiter's position in relation to the sun, you could take your noon reading and then wait for the moment that Jupiter rises. At that moment, you can determine your new local time by how much time has elapsed from the sun at high noon and Jupiter at the horizon. That should give you what you need to compare both times (local and port) at the same moment. But you'd need an unobstructed horizon and the certainty that your latitude hasn't drifted.

ADDED: @ DaveC. lol! Can you imagine filling out paperwork and where it asks for profession you write, "hourglass turner". Wow. That would suck. And it's probably harder than it seems! But at least it would get you exploring the world.

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DaveC426913
Gold Member
ADDED: @ DaveC. lol! Can you imagine filling out paperwork and where it asks for profession you write, "hourglass turner". Wow. That would suck.

Sucks? You can bet your booties sailors would be stabbing each other for that job.

Ever hear any sailor volunteering for the jobs of midnight watch in the Crow's Nest (a great place for frostbite)? Barnacle-scraper? Poop deck scrubber? Sailor's lives were worth slightly more than rats.

One must take into account the distance from Jupiter the observer on Earth is. If Earth is in conjunction with Jupiter, the events near Jupiter will occur about 16 minutes later than events which occur when Jupiter is in opposition

DaveC426913
Gold Member
One must take into account the distance from Jupiter the observer on Earth is. If Earth is in conjunction with Jupiter, the events near Jupiter will occur about 16 minutes later than events which occur when Jupiter is in opposition
Yes, presumably that would all be in the tables provided to the ship's captain.

One must take into account the distance from Jupiter the observer on Earth is. If Earth is in conjunction with Jupiter, the events near Jupiter will occur about 16 minutes later than events which occur when Jupiter is in opposition
Still, this is interesting info for a novice like myself. It seems odd, though. I'd think if Jupiter was closer, i.e. conjunct, then events would happen faster or sooner. After all, the light doesn't have to travel as far. Anyone care to explain why a conjunction makes events happen later? Maybe I'm not thinking hard enough. I'll keep mulling it...

I was using the Sun as the reference; conjunction here being the Sun, Earth, and Jupiter in that order. Opposition being Earth, the Sun, and Jupiter.

Hi Jim! Yes, I understand that the sun is the reference point in this conjunction. But there is still less distance for the light to travel in this scenario. In the conjunction, there is only the distance from Earth's orbit to Jupiter's orbit. But in the opposition, the light has to travel that same distance PLUS the distance of the diameter of Earth's orbit.

Maybe you had it backwards? I know that it takes about 8 minutes for the sun's light to reach us, so 16 minutes is the time it takes for light to travel the entire diameter of Earth's orbit. So in a conjunction, we would see events 16-minutes SOONER that the opposition.

You must've just written it out backwards. An easy mistake! :-)

You are correct! I had it backwards. Twice!